1. Field of the Invention
The invention relates to a particle-optical instrument, comprising a particle source for producing electrically charged particles which move along an optical axis of the instrument, an energy filter with an optical axis, an initial portion and an end portion of which coincide with the optical axis of the instrument, which energy filter comprises successively a first, a second, a third and a fourth sector magnet, the distance between the first and the second sector magnet being equal to the distance between the third and the fourth sector magnet, the optical axis being mainly circular with a radius of curvature R.sub.0 within said sector magnets, and also comprising a first correction magnet for generating a hexapole field between the first and the second sector magnet, and a second correction magnet for generating a hexapole field between the third and the fourth sector magnet.
2. Description of the Related Art
An energy filter for use in a particle-optical apparatus as described above is known from a publication in "Optik", Vol. 73, No. 2 (1986), pp. 56-68, entitled "Test and improved design of a corrected imaging magnetic energy filter", by S. Lanio and H. Rose. The cited article, notably section 3.2, describes an energy filter in which a number of second-order image aberrations are corrected by means of magnetic hexapole fields.
In a particle-optical apparatus such as a transmission electron microscope, particles (electrons) can be inelastically scattered by a specimen to be examined. Generally speaking, these electrons do not contribute to image contrast, but cause noise or a reduction of contrast in the specimen image formed by the microscope. Elimination of such electrons from the image-forming electron beam will result in improved image quality. This can be realised by means of an energy filter in which a selection slit is arranged in an energy-dispersive location on the optical axis in order to select a given energy window. In some cases it is also important that the local composition of a specimen can be determined. To this end, use can be made of an imaging spectrometer whereby the energy distribution in the imaging electron beam can be measured; the energy spectrum thus measured provides information regarding the structure of the specimen point considered and also regarding its chemical composition. Furthermore, it is sometimes desirable to select a given band from the energy spectrum in order to form an image exclusively by means of the particles of this energy. For both cases, i.e. for energy selection as well as for spectrometry, use can be made of a spectrometer in which the electrons successively traverse a first, a second, a third and a fourth magnetic field. Each of these fields is generated by a sector magnet in which the path of the electrons is shaped as a sector of circle. The dispersive effect of the filter is achieved by these magnetic fields.
A non-inverting filter which can be readily built into the column of an electron microscope can be constructed by using four magnetic fields. A filter with dispersive magnetic fields exhibits image aberrations which reduce the quality of the filter. A number of image aberrations can be corrected in known manner by introducing symmetry in the arrangement of the sector magnets or by imparting a given curvature to the poles of the magnets. From the cited article it is also known to correct the geometrical second-order aberrations by means of magnetic hexapole fields. To this end, the cited article proposes to generate hexapole fields between the first and the second sector magnet as well as between the third and the fourth sector magnet. These hexapole fields enable correction of a number of second-order geometric image aberrations, but not all second-order image aberrations. Notably the second-order chromatic aberration in the direction of dispersion (also known as the "oblique spectrum plane") is not reduced by this correction method. Consequently, deviations from the ideal beam path occur notably in the energy selection plane. Therefore, the use of the known filter is not very attractive for spectroscopy purposes.